Yogurt is a product which results from incubating milk, partly skimmed milk, or skimmed milk with a yogurt culture with or without added non-fat milk solids. This cultured dairy food is made by fermenting milk with cultures of the microorganisms Lactobacillus bulgaricus and Streptococcus thermophilus. This fermentation results in a spoonable custard-like milk product having somewhat of a firm body similar to light bodied pudding. Yogurt has a clean, distinct acid taste and cultured milk flavor resulting from the fermentation process.
Prior development and technology in yogurt making processes has resulted in yogurt with lower calories. The methods previously utilized for producing low calorie yogurt have included the reduction or elimination of sweeteners replacing them with artificial non-nutritive sweeteners such as aspartame and saccharin.
Reducing the caloric content of yogurt even further can result in a product having a weak body and/or poor flavor if it involves an extreme reduction in the overall solids of the yogurt. A simple reduction in the milk solids not only results in a weak bodied product but also one that is nutritionally inferior, especially with regard to the protein content.
In the conventional process of making yogurt, pasteurized milk, either with or without added non-fat milk solids, is inoculated with a culture of yogurt organisms and incubated at a temperature of 104.degree. to 113.degree. F. The growth of the organisms at the incubation temperature produces lactic acid. When sufficient acid has been formed to lower the pH of the incubating admixture to about 5.0, the casein in the admixture begins to flocculate and form a gel. Bacterial growth and acid development continue in yogurt manufacture until a pH in the range of about 4.5 to 3.5 is reached. In plant operation, satisfactory incubation or bacterial growth usually requires about three to seven hours. In order to preclude further acid development and bacterial growth in the yogurt product resulting from the incubation step, it is refrigerated at a temperature in the range of 30.degree. F. to 50.degree. F. Refrigeration not only stops the growth of the organisms but also firms up the resulting yogurt gel.
Yogurt can be sweetened without increasing its caloric content by either of two routes. The lactose present in a given yogurt product can be subjected to enzymatic digestion by lactase, resulting in the production of sugars such as glucose which impart greater sweetness. Lactase or beta-galactosidase or beta-galactopyranosidase is a very specific enzyme which splits the disaccharide lactose into the two monosaccharides, glucose and galactose. It is desirable that the lactase be free of other enzymes which would be active in the milk system, such as proteases which could cause off-flavors and enzymes which would result in fermentation of the glucose with the formation of alcohol.
The hydrolysis of a lactose in the presence of a lactase enzyme is known. The Turnbow patent No. 1,737,101, which was issued in 1929, describes hydrolysis of lactose by lactase enzyme to produce sugars which are more water-soluble than lactose. This feature allows incorporation of more milk solids and production of frozen milk products having improved quality.
Alternatively, increased sweetness is conventionally achieved by the addition of an artificial sweetener such as aspartame, cyclamate or saccharin to the yogurt.
As will be shown hereinafter, added sugar can be entirely eliminated from sweetened yogurt by using ultrafiltered lactase treated skim milk and an artificial sweetener. This aspect magnifies the desirable attribute of reducing calories in a sweetened yogurt since in total absence of added sugar, the caloric reduction by the present invention is up to 33%.
It is frequently difficult or unsatisfactory to apply the first technique of sweetening, i.e., enzymatic digestion of lactose, to a diluted ultra-filtered skimmed milk because of its low lactose content. However, alternatively leaving the lactose level high produces a product which is not digestible by lactose intolerant ethnic groups or individuals.
A significant proportion of the world's population is unable to digest the disaccharide lactose due to a deficiency of lactase in the intestinal mucosa. Approximately 60-90% of non-Caucasian people have low lactase activity. This inability to digest lactose may result in the manifestation of symptoms such as stomach cramps, flatulence and diarrhea when milk and certain other dairy products are consumed. Commercial yogurts frequently contain appreciable amounts of lactose due to the practice of fortifying the yogurt mix with non fat milk solids.
In addition, the market for yogurt is somewhat limited in that many people do not enjoy its characteristic sharp acid flavor. Attempts to improve the flavor of yogurt made in the conventional way have not been entirely satisfactory. The addition of sugar and flavors, particularly fresh pureed fruits and berries, to the mix undergoing the incubation stage has met with disfavor in that the incubation conditions cause loss of flavor and color in the flavoring additives.
It is known to prepare cultured milk products by membrane filtration of milk and similar liquid milk products containing coagulable protein to concentrate the protein content before the protein is coagulated by incubation. The pressure applied to effect membrane filtration cannot exceed the osmotic pressure of the solution. Where this pressure is considerable, such as in solutions of solutes of similarly small dimensions, the process is usually known as reverse osmosis. When solutions of substantially bigger and generally organic molecules such as proteins are involved, the term ultrafiltration is usually applied to the process.
In Chemical Engineering Publication "Ultra-filtration: An Emerging Unit-operation" May 9, 1978 at page 170, second column, ultrafiltration is taught for concentration of proteins in skim milk for the production of a low fat yogurt.
The French Patent Publication, FR No. 2,224,096, relates to the preparation of yogurt from ultrafiltered milk said to have high protein and low lactose contents. The invention of the French patent is directed to improvement of the storage life and stability of yogurt by a reduction in the fermentable lactose content, which reduces the content of lactic acid formed by lactose fermentation.
The Kosikowski U.S. Pat. No. 4,689,245 issued Aug. 25, 1987 relates to a coffee whitener composition made from milk ultrafiltration retentate. Kosikowski et al. disclose an ultrafiltration process which utilizes a membrane which passes water, lactose and other soluble milk components but retains milk proteins and fats (column 2, line 15). Kosikowski et al. do not disclose a yogurt making process.
The Henson U.S. Pat. No. 4,228,189, issued Oct. 14, 1980, relates to the preparation of yogurt utilizing a semi-permeable membrane to reduce minimal salt content and concentrate protein content of milk. Henson et al. indicate that ultrafiltration connotes the application of a semi-permeable membrane to separate solutions of large organic molecules such as proteins (U.S. Pat. No. 4,228,189, column 1, line 40). In the example, Henson, et al. discloses concentrating protein in fresh milk with a membrane; however, the lactose concentration remained unchanged after application of the membrane.
The Canadian Patent, CA No. 197711 relates to a naturally sweetened yogurt having a low calorie content. The patent discloses the addition of a lactase enzyme to a milk product in order to convert lactose in the yogurt to simple sugars which are more soluble in water and are also sweeter than lactose. Glucose and galactose are the principle sugars.
Other publications that disclose the use of lactase treated milk in yogurt production include:
Engle, W. G. 1973. "The Use of Lactase to Sweeten Yogurt Without Increasing Calories". Cult. Dairy Prod. J. 8:6; and PA1 Leary, V. S. and J. H. Weychik. 1976. "A Comparison of Some Chemical Properties of Yogurts Made From Control and Lactase-Treated Milks." J. Food Sci. 41:791.
The PCT Patent Publication WO No. 88/09125 relates to a low calorie, low fat yogurt product prepared by a process which included adding a nutritive sweetener. Claims 6 and 7 recite aspartame as a nutritive sweetener.
The Wolkstein U.S. Pat. No. 4,626,441, issued Dec. 2, 1986, relates to the use of aspartame in dietetic frozen yogurt or dietetic yogurt comprising: as the sweetener, mixtures of aspartame with synergistic sweetener or aspartame alone; in combination with yogurt cultures and a bulking agent. Wolkstein further discloses the addition of lactase to convert lactose to glucose and galactose. See column 2, line 47.
The Malone U.S. Pat. No. 4,430,349 issued Feb. 7, 1984, relates to an artifically sweetened yogurt prepared by mixing a stabilizer solution containing high methoxyl pectin, low methoxyl pectin and aspartame with prepared yogurt.
An object of the present invention is a process for the production of low calorie, low lactose yogurt.
It is a further object of the present invention to produce a low calorie, low lactose yogurt by a combination of ultrafiltration, enzymatic cleavage of lactose, and addition of an artificial sweetener.
The present invention is related to the invention described in copending patent application U.S. Ser. No. 413,200 filed Sept. 27, 1989 and assigned to the same assignee as the present invention. The invention of U.S. Ser. No. 413,200 is directed to the use of added fiber in yogurt to reduce calories and increase dietary fiber. The U.S. Ser. No. 413.200 application is incorporated herein by reference, and the techniques of cooking the fiber with a fruit filling and adding the cooked fiber and fruit to yogurt is also operative in the present invention.